Operational Amplifier

              

Voltage Follower (also known as a buffer amplifier)

Passive Low Pass Filter

f = 1 / (2*Π*R*C)            

Example:
R = 10K
C = 10μF
f = 1 / 2 * 3.1416 * 10000 * 0.00001
f = 1.6Hz

Active Low Pass Filter

f = 1 / (2*Π*R2*C)           

Passive High Pass Filter

f = 1 / (2*Π*R*C)           

Example:
R = 1K
C = 1nF
f = 1 / 2 * 3.1416 * 1000 * 0.000000001
f = 160kHz

Active High Pass Filter

Envelope Detector